The present invention relates to a V-type four stroke engine, and in particular to a V-type four stroke engine which has a bank angle which is other than 90.degree..
In the four stroke multi cylinder internal combustion engine art, it is per se known to so configure the cylinders and the crank as to make the firing of the cylinders evenly spaced with respect to crank angle, and to keep the low order components of the torque fluctuation low. For this even firing, in a V8 engine in which the crankpins are formed as double crankpins each of which serves for two cylinders, it is required for the bank angle between the cylinder banks to be 90.degree.; and, on the other hand, in a V8 engine in which the bank angle is not 90.degree., it is required to arrange the inter-bank angle between corresponding crank pins to be equal to (90.degree.-the bank angle). For further information on this matter, reference should be made to the issue of the journal Jidoosha Koogaku Benran dated 1 Dec. 1976, or to U.S. Pat. Nos. 3,308,680 and 3,978,828.
In such a case as that outlined above, i.e. in a V8 engine in which the bank angle is not 90.degree. and the inter-bank angle between corresponding crank pins is equal to (90.degree.-the bank angle), the cylinders fire evenly although the bank angle is not 90.degree.; but, as compared with the 90.degree. case, the unbalanced moment due to reciprocating and rotating parts is increased, and vibration is increased. Therefore in practice at the present time, due to this problem, the four stroke non 90.degree. V8 engine is not used for automobiles. And similar considerations apply to a V6 type engine. Yet, the provision of such a non 90.degree. V type engine would be very convenient in terms of mounting convenience in a modern type vehicle; in particular, it would be very desirable to reduce the bank angle of a V8 type engine to be less than 90.degree., in view of the evident desirability of reducing the total width of the engine. Such considerations are becoming more and more important nowadays, in view of the ever increasing requirements for reducing the exterior size of the vehicle and accordingly for restricting the size of the vehicle engine room.